Universal Serial Bus (“USB”) technology is designed to provide electrical power from hosts to devices connected to the hosts by a USB cable. In particular, a host USB receptacle draws electrical power from the host power supply and provides electrical power to a USB plug mated with (“plugged into”) the receptacle with power carried from one plug at the host end of a USB cable to another plug at the device end of the USB cable mated with a device receptacle. Some hosts, e.g., many types of consumer electronics devices and furniture (e.g., tables, lamps, alarm clocks, etc.), are designed to provide power to and/or charge electronic devices over a USB cable. In particular, a power supply provides power to the host and the host provides some of this power to the device over a USB connection. When used in this way, consumer electronic devices and furniture serve as “charging stations” for electronic devices, e.g., mobile phones, tablet computers, audio players, video players, etc., plugged into the host.
Many conventional USB charging receptacles draw power from a host power supply even when the charging receptacle is empty, i.e., when the USB charging receptacle is not mated with a USB plug (see, e.g., FIG. 1). Consequently, electrical power is provided to USB charging receptacles in the absence of a device needing a charge or needing power from the host. This use of electrical power when not needed is wasteful and can be inconvenient. As an example, for some uses, consumers provide electrical power to hosts from a battery, e.g., to eliminate the need to provide electrical power to the host over a power cord, e.g., due to the lack of convenient household (e.g., alternating current) outlets, to eliminate a physical (e.g., tripping) safety hazard, or to remove an unsightly electrical cord for aesthetic reasons. Due to the design of standard USB technology, the power drawn by USB receptacles from battery-powered hosts in the absence of plugged-in devices can result in depleting the battery, often resulting in a non-functional host and/or a host that cannot provide power to connected devices until the battery is recharged. Further, while the power (be it battery or direct/alternating current) wasted by one unused USB charging receptacle is small, the aggregate power wasted over time is enormous, e.g., costing many millions of dollars each year in wasted energy consumption.
Some previous technologies have attempted to address this problem using an external, separate switch to control electrical power provided to the USB receptacle. However, an external switch potentially lacks efficiency because use of the switch requires a user to move the switch to the “off” position to interrupt power to the USB charger receptacle and then to move the switch to the “on” position to provide power to the USB charger receptacle. If the user forgets to move the switch to the “on” position, a device plugged into the charging receptacle will not charge. Further, if a user forgets to move the switch to the “off” position when charging of a device is not needed, the USB charger receptacle will waste power and, in cases of battery-powered hosts, the wasted power will drain the battery. Another technology designed to address this problem—named the “Universal Serial Bus Power Delivery Specification”—uses a current-sensing circuit to determine if a device is plugged into a USB charging receptacle before enabling the USB charging circuit. This technology also lacks efficiency because the current-sensing circuit places the USB charging receptacle into a “sleep state” rather than completely interrupting power to it. This technology periodically provides power to the USB charging receptacle to determine if the receptacle is mated with a plug, that is, to determine if a device has been plugged in. Although the current sensing circuit may extend battery life, the current-sensing technology consistently draws power from the host, thus wasting electrical power and, in cases of battery-powered hosts, the wasted power drains the battery.